Modern quantum chemical methods can be employed for theoretical calculations of the electronic structures of representative Mn-oxo and Fe-oxo complexes relevant to biology. Quantum mechanical density functional methods are used to describe and analyze important properties of the active site. Recently, our group combined the density functional approach with an electrostatic description of the longer range environment. Our purpose is to develop a detailed understanding of chemical bonding, magnetic properties, and energetics of these systems, and to relate these to the underlying electronic structure as a function of oxidation state, ligand environment and geometry. Major goals of this project are: (1) To perform calculations on Mn-oxo dimer and tetramer complexes to characterize different oxidation states and coordination geometries that may be relevant for water oxidation and molecular oxygen evolution as occurs in the oxygen evolving complex (OEC) of photosystem II in plants and cyanobacteria. (2) To perform electronic structure calculation on well defined model Fe-O dimer complexes containing aquo, hydroxo, oxo and carboxylate bridging groups, and terminal nitrogen and/or oxygen ligands. Comparisons will be made of calculated magnetic and spectroscopic properties with current experimental studies of synthetic Fe-O systems, and with calculated and experimental properties of enzyme active sites including ribonucleotide reductase (RR) and hemerythrin (Hr), and methan monooxygenase. Quantum mechanical geometry optimization will be used on active site and synthetic model systems. (3) to further develop and apply the coupled density functional/electrostatic methodology on large spin coupled transition metal complexes containing manganese-oxo and iron-oxo centers.